saplings now only check for groups, not specific nodes, when determining
whether they are supported. the previously checked nodes are now in the
new group that's being checked.
make the wrapper `blockgame.register_node` make reasonable assumptions
that make node registration more convenient.
change other code to take advantage of that change.
also remove the `blockgame.reg_simple_node` utility since it's now
redundant.
create node representing growing leaves, and use it where previously
living leaves was used. this allows for a distinction between leaves
that will try to create more leaves, leaves that can provide nutrients
to other leaves, and leaves that are dead, whereas previously the first
2 of those couldn't be distinguished.
move plank registration and recipe out of tree mod into newly created
woodworking mod.
also update plank recipe to require cobblestone instead of stone to be
held while pummeling.
add `log_alive` and `leaves_alive`. make alive logs actually have a top
texture (bottom texture and dead logs are still wrong, though).
slightly refactor some tree growth code.
